1. Field of the Invention
The present invention relates to an air flow rate measuring device of the type which employs a heat-generating resistor and, more particularly, to an air flow rate measuring device of the type described, suitable for use in measuring intake air flow rate in an automotive engine to obtain air flow rate information to be used in the control of fuel injection performed in such an engine.
2. Description of Related Art
A conventional air flow rate measuring device of the type using a heat-generating resistor is disclosed in, for example, Japanese Patent Unexamined Publication No. 2-232524. This conventional device includes a sub-air passage having a recessed inlet and orthogonally bent at a portion downstream of a flow rate measuring portion so as to open in a joining outlet provided in the vicinity of the wall of a main air passage, thereby diminishing output variation in the air flow rate measuring device caused by an offset of flow of the air occurring in the region upstream of the air flow rate measuring device.
This arrangement, however, cannot satisfactorily reduce error or deviation of the measuring output to the minus side caused by offset of air flow of small flowing velocity appearing in the region of the sub-air passage upstream of the flow rate measuring portion, although the reduction in such an error is not absolutely impossible. In addition, the above-described arrangement has not been considered to eliminate influence of an air cleaner or other component which would cause such an offset of the air flow that the velocity is high in the region between the region of the sub-air passage upstream of the flow rate measuring portion and the center of the main air passage. Thus, the above-mentioned known arrangement cannot satisfactorily reduce error or deviation of the measuring output to the plus side which is caused when the above-mentioned offset of the air flow exists.
Thus, the known air flow rate measuring device is not designed for providing sufficiently high measuring accuracy to a variety of arrangement of the conduit upstream of the air flow rate measuring device and a variety of configurations of air cleaner, and is adapted only to a specific configuration of the air cleaner.
Thus, the known air flow rate measuring device can provide measuring accuracy only when it is combined with specific configurations of the conduit upstream of the measuring device. Hitherto, no air flow rate measuring device has been proposed which would provide sufficiently high measuring accuracy adapting to various designs of air cleaner and air duct of automotive engines, through elimination of influence of offset of the air flow, in particular such an offset that the velocity is highest in the central portion of the main air passage as in the case where a straight duct is adopted.